This invention relates generally to an apparatus and method for testing a three-phase alternating current (a.c.) electric motor during production and more particularly concerns an apparatus and method for determining the performance parameters of a three-phase a.c. electric motor without the necessity of mechanically coupling the shaft of the electric motor to a dynamometer or other instrumentality during the on-line production test procedure.
In connection with the testing of a three-phase a.c. electric motor, there are a number of performance parameters that characterize the operation of such three-phase a.c. electric motor. Those parameters may include the following full load operating parameters: Efficiency, input current, input power, output power, output torque, speed, slip, power factor, iron (hysteresis) power loss, secondary rotor power loss, primary winding power loss, stray load power loss, and friction and windage power loss.
One performance parameter of a three-phase a.c. electric motor is its efficiency. Efficiency of an electric motor is the percentage of mechanical output power at the shaft of the electric motor as compared to the electrical input power for the electric: motor. Such efficiency as well as other performance parameters are conventionally determined by coupling the shaft of the electric motor under test to a dynamometer and measuring the mechanical output power with the dynamometer and measuring the electrical input power with a power meter while the electric motor is running at various loads up to its rated full load. While testing the efficiency and other performance parameters of an electric motor using a dynamometer is well known and accepted in the art, such dynamometer testing does not lend itself to testing the efficiency of each and every three-phase a.c. electric motor on-line during the manufacturing process. Particularly, efficiency testing using a dynamometer not only requires the expense of an on-line dynamometer but also involves the additional time required to mechanically couple the motor under test to the dynamometer.
Another way of determining the efficiency and other performance parameters of a three-phase a.c. electric motor is to determine those performance parameters by means of the "Equivalent Circuit Method" for testing three-phase a.c. electric motors as set forth in Japanese Test Standard JEC-37 (1979), Section 8.5 which is incorporated herein by reference. The performance parameters are calculated in accordance with the "Equivalent Circuit Method" from the direct current (d.c.) resistance of the field windings of the motor, from the data gathered from a conventional no load test (described in Section 7 of Japanese Test Standard JEC-37 (1979)), from the data gathered from a conventional locked rotor test (described in Section 6 of Japanese Test Standard JEC-37 (1979)), and from the stray load power loss of the motor.
With regard to the d.c. resistance of the primary or field windings of the three-phase a.c. electric motor, the d.c. resistance can be determined by the following two steps: (1) Measure the d.c. resistance of the primary winding while the winding is at a known temperature; and (2) adjust the measured d.c. resistance to final full load operating temperature by multiplying the measured d.c. winding resistance by the ratio of (234.5+design operating temperature) / (234.5+known temperature) for copper windings or (225+design operating temperature) / (225+known temperature) for aluminum windings. From the d.c. resistance of the field windings a full load primary winding power loss can be calculated by multiplying the square of the full load input current by the d.c. resistance of the field windings.
In the conventional no load test, the motor is connected to a source of a.c. voltage and operated with the motor shaft unconnected to any external load. Three data points are taken at approximately 100%, 50%, and 35% of rated voltage. For each data point, the input power is measured and recorded. These data points are use to obtain the friction and windage power loss as described below and to provide the no load data points (100% and 50%) required in calculating certain performance parameters.
In the conventional locked rotor test, the motor's shaft is mechanically locked to prevent rotation. A reduced a.c. voltage, at the rated frequency, generally 60 Hz, is connected to the field windings to produce the rated current in the field windings. The electrical input power and input voltage are measured and recorded. The frequency of the input voltage is then reduced to between 15 Hz and 20 Hz, and the electrical input power and input voltage are measured and recorded again. These two data points are also used in the "Equivalent Circuit Method" for calculating certain performance parameters.
The full load stray load power loss is the loss which cannot be accounted for in any of the other losses (i.e. friction and windage power loss, primary power loss, secondary rotor power loss, and iron (hysteresis) power loss) and is therefore required in order to calculate other performance parameters of the motor accurately. The stray load power loss is in essence a characteristic of the motor design and is determined in conventional fashion using a dynamometer. The full load stray load power loss, once determined using a dynamometer on the motor, can then be used as a constant for a particular motor design in subsequent testing of other motors of the same design without the further need for dynamometer testing.
From the test data generated by the no load test, the friction and windage power loss, one of the performance parameters, can be determined. Obviously, all the power needed to run the motor at no load is lost. As the input voltage approaches zero the input power approaches the value of the friction and windage power loss. In addition, the no load data along with the primary winding power loss forms a basis for determining the iron (hysteresis) power loss for the motor. Graphically, the calculation of the friction and windage power loss and the iron (hysteresis) power loss can be represented by plotting the no load data points on a graph having power along the y-axis and voltage along the x-axis. The resulting plot can then be extrapolated to intersect the y-axis. The value of the intersection of the y-axis represents the friction and windage power loss. The iron (hysteresis) power loss is the difference between the no load electrical input power at the rated voltage and the friction and windage power loss minus the primary power loss at the rated no load input voltage.
Secondary rotor power loss is a performance parameter that may be of interest in evaluating the performance of a three-phase a.c. motor. The secondary rotor power loss is the loss resulting from the d.c. resistance of the rotor. In a three-phase a.c. motor with a squirrel-cage rotor, the secondary d.c. resistance cannot be measured directly. Because the secondary d.c. resistance and slip are related, the conventional method of obtaining full load secondary rotor power loss is determined by using the equation: EQU 2ndIIR=slip x (INPW-F&W-Fe-PriIIR)
where:
slip=(sync. rpm-full load rpm)/sync. rpm; PA0 INPW=total input power in watts at full load; PA0 F&W=friction and windage power loss; PA0 Fe=full voltage iron (hysteresis) power loss; and
PriIIR=full load primary winding power loss.
Several other performance parameters can also be calculated from the no load data, the locked rotor data, the stray load loss data, and the d.c. resistance of the primary field windings using the "Equivalent Circuit Method". The performance parameters calculated from the collected data includes: Efficiency, full load input current, full load input power, full load output power, full load output torque, speed, power factor, iron (hysteresis) power loss, friction and windage loss, the primary winding power loss, and secondary rotor power loss.
While it can be seen that the efficiency and other performance parameters of a three-phase a.c. electric motor can be determined by using the data from off-line stray load loss testing, from no load testing, from locked rotor testing, and from determination of the d.c. resistance of the primary field windings, the necessity of mechanically locking the rotor of the motor in order to perform the locked rotor test is undesirable for on-line testing of electric motors during the manufacturing process where each and every motor is to be tested for efficiency as well as other performance parameters.